Described below is a method for transmission of a transmission signal, in particular of a mobile radio signal.
Modern mobile radio standards make use of high-quality modulation methods, such as QPSK (Quadrature Phase Shift Keying), 8PSK (Phase Shift Keying) or 16QAM (Quadrature Amplitude Modulation). In these modulation methods, symbols are transmitted with more than two possible information states. In this case, constancy of the instantaneous transmission power is dispensed with in favor of the higher data transmission rates.
Even greater variation of the instantaneous transmission power results from the use of multicarrier systems. In multicarrier systems, for example mobile radio standards based on OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access), or multicarrier GSM (Global System for Mobile Communication), a plurality of modulated individual carriers are combined to form one overall signal.
Depending on the modulation method and the combination of the individual carriers, the maximum instantaneous power of a transmission signal in multicarrier systems may therefore be considerably greater than the average value of the transmission power. The ratio of the maximum to the average transmission power is referred to as the peak to average ratio (PAR) or peak to average power ratio (PAPR) (PAR 1> and PAPR>1).
If the PAR is relatively high, the transmission amplifier in a transmission system has to keep considerable drive reserves available in order to avoid non-linear distortion of the transmission signal, for example because of limiting (clipping) effects. This increases the complexity and the power consumption of the amplifier; furthermore, the efficiency of the overall system is reduced. This in turn limits the available transmission power and range of the transmission device.
In order to reduce the peak power, it is possible to use methods in which the amplitude of signal peaks is limited (clipping). This adds an error to the signal, which becomes greater the greater the extent to which the peak power is reduced. Reduction methods such as these can be assisted by intelligent transmission modes in which transmission capacity is deliberately kept free or is filled with unusable signals, such that the PAR falls.
In order to restrict the requirements for the transmission amplifier, DE 102 19 318 A1 discloses a method for forming a carrier-frequency output signal with n signal elements in a base frequency band, in which each individual one of the n signal elements is respectively associated with one main branch as an input signal and is amplitude-limited, filtered and converted to an intermediate-frequency signal element. The intermediate-frequency n signal elements are combined as output signals from the main branches to form a joint sum signal. The sum signal is amplitude-limited, converted from digital to analog, and converted to the carrier-frequency output signal. In addition, each signal element is passed into a secondary branch, with the aid of which signal peaks to be expected in the signal element in the corresponding main branch are determined as estimated values. Amplitude limiting processes which are in each case carried out in the main branch of the corresponding signal elements in the main branch are controlled with the aid of the estimated values.